Projecting patterned or structured light onto an article is a well known technique for obtaining three-dimensional information pertaining to the article. As shown in FIG. 1, a projector 1 is used to project a pattern of light, such as a series of parallel lines 2, onto surfaces 4, 6. The axis 3 of the projector 1 is oriented at an angle with respect to these surfaces 4, 6. When the lines 2 are projected onto a surface 4 that is raised with respect to another surface 6, the lines 2 appear to shift laterally between these surfaces 4, 6 when viewed from above, for example, using camera 8 and monitor 9. The magnitude of the lateral shift between the lines 2 on surfaces 4, 6 yields information about the distance between the surface 4 and the surface 6. For example, the lateral shift between the lines 2 and the angle of projection can be used to calculate the height of the surface 4 with respect to the surface 6 using triangulation.
Existing systems and methods for three-dimensional inspection using projected light patterns, however, do not adequately provide an accurate inspection of electronic packages having specular surfaces, such as Ball Grid Array (BGA) devices. Accurate inspection of electronic packages and other such articles requires high resolution measurements of the lateral shift in the lines or pattern projected onto the article. If the projected pattern or image is not properly focused or is distorted, measurements of the lateral shift in the lines of the projected pattern may not be accurate. In the existing systems having an angled projector 1, the projected image may not be in focus if the Scheimpflug condition is not satisfied, as will be discussed in greater detail below. Blurring of the lines in the projected pattern also typically occurs as the lines move away from the focus of the projector 1. As a result, the width of the lines projected onto the article may not be consistent over the entire range of the article being inspected. The width and spacing of the projected lines can also vary as a result of an effect commonly referred to as keystoning, as will be described in greater detail below.
Existing patterned light projectors also encounter problems as a result of specular surfaces, such as the solder balls on BGA devices. The reflection of light from specular surfaces often causes a saturation of pixels in the camera and necessitates the use of cameras with high dynamic ranges or logarithmic responses. Also, if a series of lines or a similar pattern is projected with a spacing equal to the spacing of the solder balls on a BGA device, light will reflect between neighboring solder balls. This type of reflection will adversely affect the image detected by the camera and thus will result in an inaccurate measurement of the shift in the lines. Furthermore, when the article being inspected has a surface and surface objects with different reflectivities, such as the solder balls and the substrate of a BGA device, it is difficult to view both surfaces with a single exposure without losing information on one of the surfaces by either saturating one of the lines or causing one to be in the noise.
Accordingly, a need exists for a system and method for three-dimensional inspection that projects patterned light in a manner that reduces unwanted reflection from specular surfaces, provides a projected pattern that is in focus, and allows high resolution measurements of the reflected light pattern to accurately determine three-dimensional information. In particular, a need exists for a system and method for three-dimensional inspection of BGA devices or similar articles having rounded specular surfaces and surfaces of different reflectivities.